The present invention relates to a data reproduction system for reproducing data from a recording medium used for data recording apparatus including an optical disk apparatus, a magneto-optical disk apparatus, and a magnetic disk apparatus.
For example, in a variety of fields such as recording/reproduction of image data and recording of codes for computers, attempts have been made to use an optical disk recording medium, such as an optical disk or a magneto-optical disk, for an optical disk apparatus because of its large capacity, compatibility and high reliability.
The optical disk apparatus is required to have a data recording/reproduction method with higher accuracy as a recording density increases. For example, proposed as such a data recording/reproduction method with higher accuracy for the optical disk recording medium is a combination of recording a recording data signal onto an optical disk recording medium by modulating the signal to a so-called partial response (PR) waveform and detecting maximum likelihood data by a so-called Viterbi detector (maximum likelihood data detector) after sampling a readout signal from the optical disk recording medium at a predetermined sampling frequency.
A basic structure of a generally known Viterbi detector is shown in FIG. 1.
According to FIG. 1, the Viterbi detector includes a branch metric calculation unit 10, an ACS (add-compare-select) unit 11, a path metric memory 12, and a path memory 13. For example, when the Viterbi detector is applied to the data reproduction system of a magneto-optical disk apparatus, the branch metric calculation unit 10 calculates branch metrics (BM) each corresponding to a difference between a sampled value yt and each of expected values of a readout signal from a magneto-optical disk. The expected values, which depend on a PR waveform employed in the recording of data, are values which the readout signal should take. When one sampled value yt is obtained, the branch metrics are calculated with respect to all the expected values.
The ACS unit 11 adds each of the above-mentioned branch metrics to a corresponding one of previous path metrics (PM), which are calculated at a previous sampling timing to be stored in the path metric memory 12. The ACS unit 11 then compares each given pair of the calculated path metrics so as to select the smaller of the two. The selected path metrics are stored in the path metric memory 12 as new path metrics. According to the above-described calculation, a path metric is expressed as a cumulative value of branch metrics. Selecting the smaller path metric as described above corresponds to selecting a state transition path. That is to say, the ACS unit 11 always selects a path so as to take the smallest path metric.
Data (binary data) corresponding to the paths selected in the above-described manner are supplied to the path memory 13 from the ACS unit 11. In the path memory 13, the data corresponding to the selected paths is shifted, while data corresponding to non-selected paths is discarded. As a result, data corresponding to a surviving path is output from the path memory 13 as detected data.
Thus, the data recorded with high density on a magneto-optical disk can be reproduced with high accuracy by recording the data in the PR waveform and detecting the maximum likelihood data by using the Viterbi detector.
If the waveform of a readout signal as shown in FIG. 2(a), for example, is obtained from a magneto-optical disk, the sampled values of the readout signal are shown in a histogram having a shape as shown in FIG. 2(b) or (c). This example shows a case of subjecting data recorded with a PR(1, 1) waveform to Viterbi detection (maximum likelihood sequence detection). In the case of considering only white noise, the distribution of the sampled values includes three peak levels corresponding to expected values as shown in FIG. 2(b).
However, if the readout signal includes a transient response, an offset variation, a phase error or a nonlinear torsion component, the sampled values are distributed irregularly as shown in FIG. 2(C). A sufficient error rate characteristic cannot be obtained by subjecting such sampled values to Viterbi detection by using the expected values fixed to constant values.
It is known that greater improvement in a data detection accuracy can be achieved with a longer constraint length of a PR waveform such as a PR(1, 2, 1) or a PR(1, 2, 2, 1). However, the longer constraint length decreases an amplitude margin per expected value, so that a data reproduction process is more easily affected by the transient response, the offset variation, the phase error or the like so as to deteriorate an error rate.
Further, in recent years, attempts have been made to put into practical use a recording medium having an MSR (Magnetically induced Super Resolution) effect in order to realize high-density recording of data. In this MSR medium, super resolution effect is produced by forming a mask using the heat distribution of a light beam. Therefore, a nonlinear torsion component is generated in a readout signal by a non-uniform heat distribution of the light beam moving on a recording medium, thus causing distortion of the waveform of the readout signal. As a result, ideal sampled values of the readout signal cannot be obtained, thus contributing to the deterioration of the error rate.
Moreover, uneven rotation of a spindle rotating an optical disk recording medium, and recording/reproduction of data by different disk drives cause phase and frequency deviations between a reference clock and recorded data. A phase-locked loop is employed to eliminate these deviations. However, a signal resolution is lowered in the case of the longer constraint length as described above, so that it becomes difficult to obtain a stable phase error signal for synchronization by binarizing the readout signal in the conventional way.
It is a general object of the present invention to provide an improved, useful data reproduction system in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a data reproduction system which allows a Viterbi detection process to be performed without being easily affected by the transient response, offset variation, phase error and nonlinear torsion component of a readout signal even though the constraint length of a PR waveform used for the recording of data onto a magneto-optical recording medium is increased.
A further object is to provide a data reproduction system in which data can be reproduced by using a more stable phase error signal for synchronization obtained from a readout signal even though the constraint length of a PR waveform used for the recording of data onto a magneto-optical recording medium is increased.
In order to achieve the above objects, a data reproduction system according to the present invention, which system determines reproduced data based on comparison results of path metrics calculated in accordance with a Viterbi algorithm based on branch metrics calculated from expected values and a sampled value of a readout signal, includes an expected value setting unit for variably setting the expected values used for a calculation of the branch metrics, the readout signal being obtained from a recording medium on which data is recorded in accordance with a recording signal of a partial response waveform, the expected values being determined by the partial response waveform, and the sampled value being obtained by sampling the readout signal at a predetermined frequency.
According to the above-described data reproduction system, since the expected values, which are values which should be obtained when sampled, can be set variably, differences between real sampled values and the expected values can be reduced by varying the expected values in accordance with the characteristic of the readout signal (an offset, a nonlinear torsion, etc.) which depends on the characteristics of the recording medium and the reproduction,system. As a result, data having more likelihood can be reproduced with higher accuracy.
The recording of data with a PR waveform of a longer constraint length requires an increased number of expected values to be set. In such a case, in the light of specifying necessary expected values with ease, the above-described expected value setting unit of the above-described data reproduction system can be structured to include an expected value specification unit which specifies expected values used for obtaining a smallest path metric every time the readout signal is sampled so as to set the expected values specified in said expected value specification unit.
The expected values used for obtaining the smallest path metric when the readout signal is sampled are the values the sampled values should take. Therefore, the expected values corresponding to the sampled values can be set easily in the above-described data reproduction system.
The above-described expected value setting unit can be structured to include a default expected value setting unit which sets default values of the expected values so as to calculate expected values to be set by correcting the default values of the expected values set in said default expected value setting unit.
The default expected value setting unit can initially set the expected values determined by the partial response waveform of the recording signal as the default expected values. From a viewpoint that more proper expected values can be set in a data reproduction process, the above-described default expected value setting unit can be structured to include a unit which sets the calculated expected values as default values used for calculating next expected values to be set.
A readout signal from an optical disk recording medium is likely to drift immediately after the beginning of the reproduction of data (a transient response). In this case, real expected values (values which should be obtained when sampled) shift from expected values determined by the partial response waveform of a recording signal. Therefore, in order to reproduce accurate data even if the readout signal drifts immediately after the start of the reproduction of the data, the above-described system can be structured to include an initial offset setting unit which sets, as an initial offset, an offset included in the readout signal obtained immediately after the start of the reproduction of the data, and to provide the expected value setting unit with a first expected value calculation unit which calculates expected values to be set based on the initial offset set in the initial offset setting unit.
According to the above-described data reproduction system, the initial signal offset setting unit sets the offset included in the readout signal obtained immediately after the start of the reproduction of the data as the initial offset, and the first expected value calculation unit calculates the expected values to be set based on the initial offset. Therefore, even though the readout signal drifts immediately after the start of the reproduction of the data (that is, the readout signal includes the initial offset), it is possible to set the expected values considering the drift component.
During the steady reproduction of a signal, a readout signal drifts in some cases depending on the characteristic of an optical disk recording medium, the specification of a data reproduction system in use, and an environment such as temperature. In such a case, real expected values (values which should be obtained when sampled) shift from expected values determined by the partial response waveform of a recording signal. Therefore, in order to reproduce accurate data even if the readout signal drifts during the data reproduction process, the above-described system can be structured to include an offset detection unit which detects an offset included in the readout signal obtained steadily, and to provide the expected value setting unit with a second expected value calculation unit which calculates expected values to be set based on the offset detected in the offset detection unit.
According to the above-described data reproduction system, when the offset detection unit detects the offset included in the readout signal during the steady reproduction of the signal, the second expected value calculation unit calculates the expected values to be set based on the offset. Therefore, even though the readout signal drifts during the steady reproduction of the signal (that is, the readout signal includes the initial offset), it is possible to set the expected values considering the drift component.
In some cases, a readout signal is distorted by double refraction generated in an optical head so as to show a nonlinearity. In this case, real expected values (values which should be obtained when sampled) shift from expected values determined by the partial response waveform of a recording signal. Therefore, in order to reproduce accurate data even if the readout signal is distorted to show the nonlinearity, the above-described system can be structured to include a nonlinear component extraction unit which extracts a nonlinear torsion component included in the readout signal, and to provide the expected value setting unit with a third expected value calculation unit which calculates expected values to be set based on the nonlinear torsion component extracted in the nonlinear component extraction unit.
According to the above-described data reproduction system, when the nonlinear component extraction unit extracts the nonlinear component included in the readout signal during the reproduction of the signal, the third expected value calculation unit calculates the expected values to be set based on the nonlinear torsion component. Therefore, even though the readout signal is distorted to show a nonlinearity, it is possible to set the expected values considering the nonlinear torsion.
In order to obtain proper expected values in each phase of a signal reproduction based on the above-described initial offset, steady offset and nonlinear torsion, which can be included in a readout signal, each of the above-described systems can be structured to include: an initial offset setting unit which sets, as an initial offset, an offset included in the readout signal obtained immediately after a start of a reproduction of the data; an offset detection unit which detects an offset included in the readout signal obtained steadily; and a nonlinear component extraction unit which extracts a nonlinear torsion component included in the readout signal, and to provide the expected value setting unit with a default expected value setting unit which sets default values of the expected values, and with an expected value correction unit which corrects the default values of the expected values set in the default expected value setting unit, based on at least one of the initial offset set in the initial offset setting unit, the offset detected in the offset detection unit, or the nonlinear torsion component extracted in the nonlinear component extraction unit.
According to the above-described data reproduction system, the expected value setting unit sets new expected values by correcting the default values based on at least one of the obtained initial offset, steady offset and nonlinear torsion component. For example, it is desirable to correct the default values based on the initial offset at a point immediately after the reproduction of the signal, and on the steady offset and nonlinear torsion component during the steady signal reproduction process. Further, especially when the readout signal has a small nonlinear torsion or a small steady offset, the default values can be corrected based only on the steady offset or the nonlinear torsion component during the steady signal reproduction process.
In order to calculate new expected values with higher accuracy by varying the default values of the expected values in accordance with a variation such as an offset or a nonlinear torsion of the readout signal, the above-described system can be structured to include a unit which sets expected values obtained by the correction in the expected value correction unit as default values to be used in a next correction therein.
When the bit arrangement of data is determined in accordance with a predetermined rule (for example, a 1/7 run length limit, or a 2/7 run length rule) during the recording of the data, reproduced data cannot have a bit arrangement other than the one determined in accordance with such a rule. Therefore, in order to achieve a data reproduction with higher accuracy by restricting the data having a bit arrangement against such a bit arrangement rule for recording the data, the above-described system can be structured to include a data restriction unit which forcibly restricts, on the basis of a bit arrangement rule during recording of the data, the reproduced data obtained based on the comparison results of the path metrics.
In order to achieve the further object of the present invention, another data reproduction system according to the present invention, which system determines reproduced data based on comparison results of path metrics calculated in accordance with a Viterbi algorithm based on branch metrics calculated from expected values and a sampled value of a readout signal, includes: a phase error calculation unit which calculates a phase error based on differences between the sampled value and expected values used for a calculation of the branch metrics; and a synchronizing clock generation unit which generates a clock signal determining sampling timings of the readout signal based on the phase error obtained in said phase error calculation unit, the readout signal being obtained from a recording medium on which data is recorded in accordance with a recording signal of a partial response waveform, the expected values being determined by the partial response waveform, and the sampled value being obtained by sampling the readout signal at a predetermined frequency.
According to the above-described data reproduction system, since the expected values are signal values which should be obtained, a difference between the real sampled value and the corresponding expected value corresponds to a difference between a timing at which a sampling should be performed and a real sampling timing. Therefore, the phase error is calculated based on the difference between the sampled value and the corresponding expected value, and a clock signal determining the sampling timings of the readout signal is determined based on the phase error.
The synchronizing clock generation unit can include a PLL circuit which adjusts the phase and frequency of an oscillation clock so as to eliminate the phase error, or a delay circuit which delays an external clock so as to eliminate the phase error.
In order to achieve the above object, another data reproduction system according to the present invention, which system determines reproduced data based on comparison results of path metrics calculated in accordance with a Viterbi algorithm based on branch metrics calculated from expected values and a sampled value of a readout signal, includes: an expected value setting unit which sets the expected values used for a calculation of the branch metrics; a nonlinear portion detection unit which detects a nonlinear portion of the readout signal; a nonlinearity obtaining unit which obtains nonlinearities included in the readout signal when said nonlinear portion detection unit detects a given nonlinear portion of the readout signal; and an expected value adjustment unit which adjusts the expected values set in said expected value setting unit based on the nonlinearities calculated in said nonlinearity calculation unit, the readout signal being obtained from a recording medium on which data is recorded in accordance with a recording signal of a partial response waveform, the expected values being determined by the partial response waveform, and the sampled value being obtained by sampling the readout signal at a predetermined frequency.
According to the above-described data reproduction system, the expected values used for the calculation of the branch metrics are adjusted based on the nonlinearities of the nonlinear portion of the readout signal. As a result, even though the readout signal shows a nonlinearity, the expected values are adjusted based on the nonlinearities with respect to the nonlinear portion, thus increasing a margin for the sampled value for reproducing proper data from the nonlinear portion of the readout signal so as to allow the data reproduction with higher accuracy.
The above-described nonlinearity obtaining unit can be structured to hold in advance a nonlinearity determined by the characteristic of a recording medium and to obtain the held nonlinearity. The nonlinearity obtaining unit can also be structured to include a nonlinearity calculation unit which calculates the nonlinearities based on the sampled value of the readout signal.
In order to achieve the above object, another data reproduction system according to the present invention, which system determines reproduced data based on comparison results of path metrics calculated in accordance with a Viterbi algorithm based on branch metrics calculated from expected values and a sampled value of a readout signal, includes: an expected value setting unit which sets the expected values used for a calculation of the branch metrics; a nonlinear portion detection unit which detects a nonlinear portion of the readout signal; a first nonlinearity calculation unit which calculates, based on the sampled value of the readout signal, first nonlinearities of a given nonlinear portion of the readout signal when said nonlinear portion detection unit detects the given nonlinear portion; a second nonlinearity calculation unit which calculates, based on the sampled value of the readout signal, second nonlinearities at a sampling point next to the detected portion of the readout signal, the second nonlinearities being smaller than the first nonlinearities; and an expected value adjustment unit which adjusts, based on the first nonlinearities calculated in said first nonlinearity calculation unit, the expected values set in said expected value setting unit with respect to the sampled value sampled at the detected portion, and adjusts, based on the second nonlinearities calculated in said second nonlinearity calculation unit, the expected values set in said expected value setting unit with respect to a sampled value at the sampling point next to the detected portion of the readout signal, the readout signal being obtained from a recording medium on which data is recorded in accordance with a recording signal of a partial response waveform, the expected values being determined by the partial response waveform, and the sampled value being obtained by sampling the readout signal at a predetermined frequency.
According to the above-described system, the expected values with respect to the sampled value of the portion of the readout signal detected as the nonlinear portion is adjusted based on the first nonlinearities, and the expected values with respect to the sampled value at the sampling point next to the portion of the readout signal detected as the nonlinear portion is adjusted based on the second nonlinearities smaller than the above first nonlinearities, thus allowing a data reproduction with higher accuracy.